M-B used these clear in-line filters in the engine compartment of some W123 & W126 diesel powered vehicles, to permit rapid visualization of engine oil in the vacuum system (due to a ruptured vacuum pump diaphragm) and as a trap for the errant oil. Central locking, engine shutoff and climate control vacuum elements are adversely affected by the presence of oil.
(Note, however, that the W126's central locking vacuum source was different from the W123, in that it used an electrically driven trunk-mounted vacuum pump, physically separate from the diesel engine's vacuum pump.)

Vehicles without the filter can be retrofitted. Brake booster vacuum line (with supply hose taps) and supply hose layout varied within the 123 and 126 series; you'll need to decide which installation location will offer the most protection for your application.